1. Technical Field
The present invention is directed toward a method of extracting oil from an oil bearing product and more particularly toward a method of extracting oil from an oil bearing product with a chilled liquefied normally gaseous hydrocarbon solvent at ambient pressures.
2. Background Art
Use of hydrocarbon solvents for removing fats and oils from various oil bearing products has been in use for many years. Hexane is one hydrocarbon solvent that has been used extensively. Hexane has the advantage of being in a liquid state under most ambient temperatures and pressures. As a result, extractions can be done without pressure resistant vessels at room or slightly elevated temperatures, which facilitates continuous and high volume extraction processes. However, hexane extraction is not without serious shortcomings. Hexane is a toxic material and may only be present in small concentrations in reduced oil content products or extracted oils for such products and oils to be safe for human or animal consumption. However, because of its relatively high boiling point (69.degree. C. at standard pressure), it is difficult to remove residual hexane from reduced oil content products and extracted oils. Hexane removal is typically accomplished by distillation, thermal flashing or stripping techniques. The relatively high boiling point of hexane means it has a relatively low volatility, approaching that of the extracted oil. The relatively close volatilities and the strong affinity between hexane and extracted oil or reduced oil content products requires relatively high temperatures and severe stripping conditions to strip residual hexane. These high temperatures and severe conditions can degrade and impair important quality characteristics such as color, taste and digestibility of both the extracted oils and the reduced oil content products and thus reduce their economic value. Moreover, even when using high temperatures and severe stripping conditions, it often is not possible to reduce hexane concentrations in a product to acceptably safe, low levels. Moreover, the separated hexane presents an air pollution hazzard which must be carefully controlled and monitored.
As an alternative, use of normally gaseous hydrocarbons under temperatures and pressures to maintain the solvents in a liquid state is known in the art. Illustrative prior art patents include Prevost, U.S. Pat. No. 5,707,673; Franke, U.S. Pat. No. 5,525,746; Hebert, U.S. Pat. No. 5,516,923; Benado, U.S. Pat. No. 5,041,245; Rosenthal, U.S. Pat. No. 2,254,245; and Reid, U.S. Pat. No. 1,802,533.
These normally gaseous hydrocarbon solvents, typically propane or butane, can be more easily separated from extracted oils and reduced oil content products than hexane, primarily because of the relatively greater difference in volatility between these normally gaseous hydrocarbon solvents and the extracted oils and reduced oil content products. While it is well known that decreasing the temperature or increasing the pressure of a normally gaseous hydrocarbon solvent can produce a liquid state, the prior art favors conducting the extractions at elevated temperatures and therefore elevated pressures. Elevated temperatures are preferred to increase the oil solubility and therefore the extraction efficiency. The prior art also teaches that conducting the extraction at elevated temperatures and pressures facilitates separation of the solvent from the extracted oil and reduced oil content products after oil extraction. However, the increased extraction pressure increases capital equipment and energy costs. Moreover, the explosive characteristics of the vapors subject to escape from such pressurized extraction vessels raises significant safety issues. Finally, the need to maintain high pressures in the extractions vessels renders continuous extraction processes very difficult and expensive to construct and maintain.
Even with the relative ease of separating normally gaseous hydrocarbon solvents from extracted oils and reduced oil content products, the prior art does not teach a suitable method for reducing the oil content of products that must maintained at low temperatures, such as frozen fried foods. Prior art methods of flashing the solvent from a reduced content oil bearing product can lead to defrosting and harm to such a product.
The present invention is intended to overcome one or more of the problems discussed above.